Differences in intestinal bile acid and steroid excretion patterns between germfree and conventional rats, and related differences in intestinal absorption of cholesterol demonstrate the importance of the intestinal microflora in steroid homeostasis. The influence of the microflora on cholesterol-bile acid metabolism will be studied: (1) by study of cholesterol levels and of the character and quantity of various bile acids in germfree and conventional rats, gerbils and dogs; (2) by introducing defined microfloras into germfree rats and studying their effect; (3) by studying the influence of various antibiotics in conventional rats; (4) by studying the influence of conditions mentioned under (2) and (3) on intestinal absorption of cholesterol, and on the catabolic conversion and elimination of endogenous cholesterol. These studies should eventually indicate the microbial species directly and/or indirectly involved in the reduction of exogenous cholesterol absorption and enhancement of endogenous cholesterol elimination caused by the presence of a "normal" intestinal microflora.